1. Field of the Invention
The present invention will be applied to a gas engine in which fuel gas introduced via a fuel supply pipe is mixed with air introduced via a charging air supply pipe and this mixture is supplied via a fuel-air mixture supply pipe to a combustion chamber of the engine. The invention relates to a method and device for controlling starting of a gas engine equipped with a fuel gas flow control valve to its fuel gas supply pipe to control fuel gas flow, a throttle valve to its fuel-air mixture supply pipe to control fuel-air mixture flow, and an electronic control unit for performing integrative control of engine rotation speed and air fuel ratio by means of the valves, specifically those of a gas engine which is improved in load responsivity while maintaining air accurate fuel ratio control and superior in starting performance with decreased starting period.
2. Description of the Related Art
Gas engines are internal combustion engines which use as fuel gaseous fuel such as natural gas. They can output high driving power with high efficiency, and widely adopted as engines for driving generators in normal and emergency service, engines for construction equipment, engines for ships, and engines for railroad vehicles. Besides, gas engines are used not only to drive generators for supplying electric power, but waste heat thereof is utilized as heat source for heating water, so they are superior in efficiency in energy use.
In a gas engine, fuel gas is supplied via a mixer into air introduced through a charging air supply pipe, fuel-air mixture consisting of the air and the fuel gas is supplied into a combustion chamber of the engine through an fuel-air mixture supply pipe, and driving power is generated by combustion of the fuel-air mixture in the combustion chamber.
In FIG. 10 is shown a conventionally prevalent gas engine. Here is shown as an example a turbocharged gas engine 1 having a subsidiary chamber for ignition.
As shown in the drawing, charging air flows through an air supply pipe 10 to a gas mixer 12, fuel gas flows through a fuel gas pipe 13, 14 to the gas mixer 12 via a main-chamber regulator 15 where air pressure is regulated and then via a main chamber fuel flow control valve 16 where fuel flow is controlled. The charging air and fuel gas are mixed in the mixer 12 to produce lean fuel-air mixture. The lean mixture is compressed by a compressor 26 of a turbocharger 25, then introduced into a main combustion chamber 7 in the suction stroke through a fuel-air mixture supply pipe 20 to be burned there after compressed in compression stroke. The burnt gas flows out from the combustion chamber 7 and is introduced as exhaust gas through an exhaust pipe 28 to a turbine 27 of the turbocharger 25. The exhaust gas drives the turbine and is exhausted outside.
On the other hand, a part of the fuel gas (subsidiary chamber fuel gas) introduced through the fuel gas pipe 13 is introduced through a subsidiary fuel gas pipe 21 branching from the fuel gas pipe 13 to a subsidiary-chamber regulator 23 where the fuel gas is regulated in pressure, then the fuel gas is introduced into a subsidiary chamber 8 provided in a cylinder head 3 of the engine 1 to be ignited by a spark of an ignition plug located at an upper position of the subsidiary chamber 8 near the top dead center of the engine cycle. The flame produced by the ignition of the fuel gas in the subsidiary chamber jets out to the main combustion chamber 7 to ignite the fuel-air mixture in the main combustion chamber.
It is necessary in the gas engine like this to control air fuel ratio in accordance with characteristics of fuel gas such as calorific value thereof in order to maintain optimum combustion evading occurrence of knocking and misfire and to reduce emission of harmful matter.
Conventionally, fuel-air mixture is controlled by the fuel flow control valve 16 to be a prescribed air fuel ratio with which normal combustion and reasonable exhaust gas property are maintained, and the fuel-air mixture of the prescribed air fuel ratio is supplied through the fuel-air mixture supply pipe 20 to the main combustion chamber 7 of the gas engine 1.
On the other hand, control of engine rotation speed is needed in order to maintain constant rotation speed in spite of changes in load. Engine speed control has been performed through controlling the flow rate of the fuel-air mixture of prescribed air fuel ratio supplied to the main combustion chamber 7 by controlling the opening of a throttle valve 18.
Conventionally, a fuel-air mixture control method consisting of air fuel ratio control and engine speed control as mentioned above has been widely adopted.
There is known another air fuel ratio control method of gas engine as disclosed in document 1 (Japanese Laid-Open Patent Application No. 5-141298). According to the disclosure, an oxygen sensor is attached to the exhaust pipe of the gas engine, and whether the fuel-air mixture supplied to the gas engine is rich or lean mixture is detected based on oxygen concentration of the exhaust gas detected by the oxygen sensor, and the air fuel ratio of the fuel-air mixture is controlled based on the result of the detection.
A further air fuel ratio control method of gas engines is disclosed in document 2 (Japanese Laid-Open Patent Application NO. 2003-262139). According to the disclosure, air compressed by the compressor of the turbocharger is introduced through an air supply path to fuel injection devices each being provided for each of a plurality of cylinders, on the other hand, fuel gas is introduced through a fuel supply path to the fuel injection devices, and fuel-air mixture mixed in each fuel injection device is supplied to each cylinder. With this control method, necessary air flow is calculated based on detected fuel flow in the fuel supply path, actual air flow is calculated based on detected air pressure and temperature in the air supply path, and air flow in the air supply path is controlled so that actual air flow coincides with calculated air flow.
On the other hand, there has been a problem in the control of gas engines that ignition of fuel-air mixture because of low calorific value of fuel gas. In document 3 ((Japanese Laid-Open Patent Application No. 6-264822) is disclosed a device for controlling air fuel ratio when starting a gas engine to ease starting of the engine. According to the disclosure, a charging air flow control valve is provided at an air cleaner attached to an inlet air passage, and the flow control valve is controlled to reduce opening thereof in order to reduce air fuel ratio to a prescribed value by reducing charging air flow when starting the engine and the valve is fully opened after the engine has started. In this way, mixture becomes easy to ignite and starting performance of the engine will be improved.
However, there has been a disadvantage that response to change of load is slow with the conventional fuel-air mixture control method as mentioned above, although it has an advantage of easiness of controlling air fuel ratio. Particularly, response when load is applied or shut off is slow, and improvement in response to load change has been demanded in order to attain high performance of gas engines. There is as one of problems of responsivity a disadvantage that, even if fine control is carried out to stabilize engine speed, stabilization of engine speed is difficult because of slow responsivity.
As a method of controlling engine speed with rapid response, there is known a method of controlling fuel gas flow to accommodate changes of load. However, with this conventional method, control of air fuel ratio is difficult, and stable combustion control cannot be achieved. As it is difficult to keep air fuel ratio in an appropriate range, there occurs a problem of compliance with exhaust emission regulation. Moreover, as fuel flow can not be detected quantitatively with the conventionally prevalent fuel gas flow control method of controlling the opening of the fuel flow control valve, over run or overload of engine due to excessive supply of fuel is apt to occur. Particularly, engine stall or abnormal combustion is apt to occur at application or rejection of load because of difficulty of accurate control of air fuel ratio when applying or shutting off load.
Furthermore, in the conventional fuel-air mixture control method, it is required to have leeway in supercharging pressure in order to secure ample engine output, and decrease in thermal efficiency is unavoidable due to pumping loss caused by throttling the mixture inlet passage to the main combustion chamber. On the other hand, with the fuel gas flow control method, the engine is immune from the problem of output shortage due to increased pumping loss, however, it is difficult to keep air fuel ratio in an appropriate range and comply with exhaust emission regulation.
With air fuel ratio control using a signal from the oxygen sensor as a feedback signal as recited in the document 1, manufacturing cost will be increased due to expensive oxygen sensor.
On the other hand, the gas engine recited in the document 2 is provided with fuel injection devices and fuel flow control valves for each of a plurality of cylinders respectively, and different from the gas engine of this patent application in basic configuration. The configuration of the gas engine of the document 1 is suited for a large engine and difficult to adopt for a small engine. Besides, as a part of air supplied from the compressor is released to outside through the air release valve to control air quantity charged into the combustion chamber, efficiency of the engine is reduced, and a larger compressor is required.
With the air fuel ratio control method disclosed in the document 3, starting performance is improved by reducing excess air ratio of the mixture, however, there occurs a problem that responsivity as mentioned above is slow when load is applied after the engine has started.
With the fuel gas flow control method, controlling at engine starting is performed by controlling opening of the fuel gas flow control valve and the throttle valve, so the engine starting is apt to be unstable.
Furthermore, with the conventional control device, air fuel ratio control and engine speed control are performed by separate control devices respectively, however, there is a disadvantage that manufacturing cost increases since the control devices are expensive, and in addition, to assure coordinated behavior of each device is difficult, which makes smooth control of the engine difficult.